NICC ND 1701 V1.5.2 ( )

Transcription

1 ND 1701 V1.5.2 ( ) Document Recommended Standard for the UK National Transmission Plan for Public Networks Network Interoperability Consultative Committee, Ofcom, 2a Southwark Bridge Road, London, SE1 9HA.

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3 3 ND 1701 V1.5.2T T( ) 2009 Ofcom copyright NOTICE OF COPYRIGHT AND LIABILITY Copyright All right, title and interest in this document are owned by Ofcom and/or the contributors to the document unless otherwise indicated (where copyright be owned or shared with a third party). Such title and interest is protected by United Kingdom copyright laws and international treaty provisions. The contents of the document are believed to be accurate at the time of publishing, but no representation or warranty is given as to their accuracy, completeness or correctness. You may freely download, copy, store or distribute this document provided it is not modified in any way and it includes this copyright and liability statement. You may not modify the contents of this document. You may produce a derived copyright work based on this document provided that you clearly indicate that it was created by yourself and that it was derived from this document and provided further that you ensure that any risk of confusion with this document is avoided. Liability Whilst every care has been taken in the preparation and publication of this document,, nor any committee acting on behalf of, nor any member of any of those committees, nor the companies they represent, nor any person contributing to the contents of this document (together the Generators ) accepts liability for any loss, which may arise from reliance on the information contained in this document or any errors or omissions, typographical or otherwise in the contents. Nothing in this document constitutes advice. Nor does the transmission, downloading or sending of this document create any contractual relationship. In particular no licence is granted under any intellectual property right (including trade and service mark rights) save for the above licence to copy, store and distribute this document and to produce derived copyright works. The liability and responsibility for implementations based on this document rests with the implementer, and not with any of the Generators. If you implement any of the contents of this document, you agree to indemnify and hold harmless the Generators in any jurisdiction against any claims and legal proceedings alleging that the use of the contents by you or on your behalf infringes any legal right of any of the Generators or any third party. None of the Generators accepts any liability whatsoever for any direct, indirect or consequential loss or damage arising in any way from any use of or reliance on the contents of this document for any purpose. If you have any comments concerning the accuracy of the contents of this document, please write to: The Technical Secretary, Network Interoperability Consultative Committee, Ofcom, 2a Southwark Bridge Road, London SE1 9HA.

4 4 ND 1701 V1.5.2T T( ) Contents Recommended Standard for the UK National Transmission Plan for Public Networks...1 Intellectual Property Rights...6 Foreword Scope Scope Purpose Application References Definitions, symbols and abbreviations Abbreviations Responsibilities and general criteria Responsibilities General criteria Reference connections Generic VPN Transmission levels General issues Telephony connections UK Loudness rating values (OLR, SLR, RLR) Voice-band data connections Signal levels sent to line Stability Delay and echo General Connections without echo control Connections with echo control National connections with echo control Special considerations for non-tdm networks The national network delay objective Networks interconnected using TDM Networks interconnected using IP Hybrid TDM/IP Call Paths Networks interconnected using other technologies International connections with echo control Echo Control Planning guidelines for the incorporation of new technologies into the UK PSTN General Rules Delay and Packet Jitter Rules Codec Rules Packet Loss Rules Post Dial Delay Jitter and wander Terms and definitions Jitter and wander in digital networks Jitter limits appropriate to digital equipments Jitter and wander tolerance of digital input ports Maximum output jitter in the absence of input jitter Jitter transfer characteristics Network synchronisation...26

5 5 ND 1701 V1.5.2T T( ) 10.1 General The synchronisation hierarchy Availability of synchronisation references Slip-rate performance Access Trunk Synchronisation for networks interconnected using IP Other parameters Error performance General issues End to end design objectives Network segment objectives Quantising distortion Background Performance objectives Noise Crosstalk Annex A (informative): Examples of describing end-to-end connections using connection elements...34 Annex B (informative): Example echo cancellation scenarios...36 History...37

6 6 ND 1701 V1.5.2T T( ) Intellectual Property Rights IPRs essential or potentially essential to the present document may have been declared to. Pursuant to the IPR Policy, no investigation, including IPR searches, has been carried out by. No guarantee can be given as to the existence of other IPRs which are, or may be, or may become, essential to the present document. Foreword This Document (ND) has been produced by End-to-End QoS Working Group.

7 7 ND 1701 V1.5.2T T( ) 1 Scope 1.1 Scope This standard provides transmission recommendations for connections (including ISDN and VPN/Centrex) carried on the public switched telecommunications network (PSTN) that are all digital from Network Terminating Point (NTP) to NTP with digital or analogue access lines from appropriate end-user terminals or private networks. These recommendations are based on assumptions about certain characteristics of the end-user terminal equipment and private networks, in particular compliance with the ETSI terminal equipment requirements set out in TBR8, TBR21, EG , EN and TBR38 (see the note in section 4.2). It is intended that these recommendations be implemented in an evolutionary manner by Operators as required by applicable services, and not to existing networks, architectures or services. The standard specifies recommended design values for network loss and other key transmission parameters. The standard also provides echo control planning and connection delay guidelines. To ensure satisfactory NTP-NTP performance to end-users, recommendations are given for the appropriate range in values for these parameters at the access interfaces to end-user terminals and interconnecting networks. Sections 7 and 8 of this document include guidelines which should be taken into account by operators planning to introduce new technologies, such as IP and ATM, into the UK PSTN. They are not intended to be applicable to all voice over IP (VoIP) applications. This document describes rules governing the end-to-end network performance of the UK PSTN and of public networks evolving towards NGN. The scope of ND1701 covers networks based on TDM, ATM and IP technology but, importantly, it does not deal with wholly NGN paths. ND1704 End-to-End Network Performance Rules & Objectives for the Interconnection of NGNs describes rules governing the end-to-end network performance of interconnected fixed and mobile UK Next Generation Networks providing PSTN/ISDN services. NGNs are based on IP technology and the interconnect between them is also IP-based. Note: The scope of ND1701 covers hybrid-technology networks and the evolutionary stages towards full UK NGN, while ND1704 cover the end picture of an all NGN UK network with NGN interconnects. ND1701 is not related to a specific NGN Release. The guidance in Clauses 7 and 8 is applicable to all NGNs, including IMS-based NGNs and NGNs that are not compliant with a specific NGN Release. 1.2 Purpose This standard provides planning rules and guidance for the evolution of networks to handle all-digital services (such as those provided by new switch and transport technologies, and data networks supporting voice), while maintaining compatibility with the current analogue and digital networks, thereby continuing high quality service to end users by the telecommunications industry. An additional purpose is to ensure correct and easy interconnection of United Kingdom Public Telecommunications Operators, as well as connections with terminal equipment and private networks for which compatible standards have been developed. 1.3 Application This standard applies to voice and data services established over fixed and mobile public networks including any connected private networks that themselves may contain VPN/Centrex services. The standard applies to connections with either analogue or digital access. Although there are many different types of access, they are classified for the purposes of this standard, in generic transmission classes illustrated in the new connection elements diagram: Copper Access

8 8 ND 1701 V1.5.2T T( ) Fixed Radio Access xdsl Access (e.g. ADSL, VDSL) GSM Access Cable Modem Access UMTS Access Either analogue or digital access connections may provide access to private networks. 2 References This standard is intended to be used with the following standards: Recommendation ITU-T Recommendation V.2 ITU-T Recommendation G.111 Table 1: ITU-T standards Description Power Levels for Data transmission over telephone lines. Blue book, Volume VIII-Fascicle VIII.1 (1988) Loudness Ratings (LRs) in an International Connection. (03/93) ITU-T Recommendation G.113 Transmission Impairments. (02/96) ITU-T Recommendation G.114 One-way Transmission Time. (02/96) ITU-T Recommendation G.121 Loudness Ratings (LRs) of National Systems. (03/93) ITU-T Recommendation G.122 Influence of National Systems on Stability and Talker Echo in International Connections. (03/93) ITU-T Recommendation G.126 Listener Echo in Telephone Networks. (03/93) ITU-T Recommendation G.131 Control of Talker Echo. (08/96) ITU-T Recommendation G.165 Echo Cancellers. (03/93) ITU-T Recommendation G.168 ITU-T Recommendation G.171 ITU-T Recommendation G.711 ITU-T Recommendation G.726 ITU-T Recommendation G. 727 ITU-T Recommendation G. 728 ITU-T Recommendation G. 728 Annex G ITU-T Recommendation G.803 Digital Network Echo Cancellers Transmission Plan aspects of Privately Operated Networks. Blue book, Volume III-Fascicle III.1. (1988) Pulse Code Modulation (PCM) of Voice Frequencies. Blue Book, Volume III - Fascicle III.4, Melbourne , 32, 24, 16 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM). (12/90) 5-, 4-, 3- and 2- bits Sample Embedded Adaptive Differential Pulse Code Modulation (ADPCM) (05/94) Coding of speech at 16 kbit/s using Low-Delay Code Excited Linear Prediction 16 kbit/s Fixed Point Specification Architecture of transport networks based on the SDH

9 9 ND 1701 V1.5.2T T( ) Recommendation ITU-T Recommendation G.810 ITU-T Recommendation G.811 ITU-T Recommendation G.812 ITU-T Recommendation G.813 ITU-T Recommendation G.821 ITU-T Recommendation G.822 ITU-T Recommendation G.823 ITU-T Recommendation G.825 ITU-T Recommendation G.826 ITU-T Recommendation G.921 ITU-T Recommendation Q.551 Description Definitions and Terminology for Synchronisation Networks. (08/96) Timing Characteristics of Primary Reference Clocks.(09/97) Timing Requirements of Slave Clocks Suitable for use as Node Clocks in Synchronisation Networks.(06/98) Timing characteristics of SDH equipment slave clocks (SEC).(8/96) Error Performance of an International Digital Connection Operating at a Bit Rate below the Primary Rate and Forming Part of an ISDN. (08/96) Controlled Slip Rate Objectives on an International Digital Connection. Blue Book, Volume III - Fascicle III.5.(11/88) The Control of Jitter and Wander within Digital Networks which are based on the 2048 kbit/s Hierarchy. (03/93) The control of jitter and wander within digital networks which are based on the SDH. (03/00) Error Performance Parameters and Objectives for International Constant Bit Rate Paths at or above Primary Rate. (08/96) Digital Sections based on the 2048kbit/s Hierarchy. Blue Book,Volume III- Fascicle III.5. (11/88) Transmission Characteristics of Digital Exchanges. (11/96)

10 10 ND 1701 V1.5.2T T( ) Table 2: ETSI standards Standard EN EN Edition 4 January 1997 ETS December 1990 EN January 2002 ETS April 1994 ES March 1998 EG V1.2.2 February 1999 EG February 1999 EG February 2000 Description Harmonized EN for CT2 cordless telephone equipment cordless telephone equipment covering essential requirements under article 3.2 of the R&TTE directive Attachment to Public Switched Telephone Network (PSTN); General technical requirements of equipment connected to an analogue subscriber interface in the PSTN (candidate NET4) Integrated Services Digital Network (ISDN); 3.1kHz telephony teleservice. Attachments requirements for handset terminals (Candidate NET33). Digital European Cordless Telecommunications (DECT) Common interface (CI) Part 8: Speech coding and transmission. Business Telecommunications (BT); Planning of loudness rating and echo values for private networks digitally connected to the public network Public Switched Telephone Network (PSTN); Requirements for Handset Telephony Speech Processing, Transmission and Quality Aspects (STQ); Overall Transmission Plan Aspects for Telephony in a Private Network Speech Processing, Transmission and Quality Aspects (STQ); Objectives and Principles for the Transmission Performance of Multiple Interconnected Networks that aim to provide "traditional quality" Telephony Services A guide to the application of TBR 21

11 11 ND 1701 V1.5.2T T( ) ETS Standard Description Transmission and multiplexing (TM); Generic Requirements for Synchronisation Networks. Part1 Definitions and terminology for synchronisation networks. (May 1998) Part2 Synchronisation network architecture (August 1999) Part3 Control of jitter and wander in transport networks.(may 1998) Part4 Timing characteristics of slave clocks suitable for synchronisation supply to SDH and PDH equipment. (June 1998) Part5 Timing characteristics of slave clocks suitable for operation in SDH equipment.(may 1998) Part6 Timing characteristics of primary reference clocks. (June 1998) Part7 - Timing characteristics of slave clocks suitable for synchronisation supply to equipment in local node applications. TBR 8 October 1998 TBR 21 January 1998 EN June 1999 TBR 38 May 1998 GTS January 1995 Integrated Services Digital Network(ISDN); Telephony 3.1kHz teleservice. Attachments requirements for handset terminals. Terminal Equipment (TE); Attachment requirements for pan-european approval for connection to the analogue Public Switched Telephone Networks (PSTNs) of TE (excluding TE supporting the voice telephony service) in which network addressing, if provided, is by means of Dual Tone Multi Frequency (DTMF) signalling. Terminal Equipment (TE); Attachment requirements for pan-european approval for connection to the analogue Public Switched Telephone Networks (PSTNs) of TE supporting the voice telephony service in which network addressing, if provided, is by means of Dual Tone Multi Frequency (DTMF) signalling Public Switched Telephone Network (PSTN); Attachment requirements for a terminal equipment incorporating an analogue handset function capable of supporting the justified case service when connected to the analogue interface of the PSTN in Europe European digital cellular telecommunication system (phase 1); Technical performance objectives (GSM 03.05)

12 12 ND 1701 V1.5.2T T( ) GTS GSM September 1996 Standard Description Digital cellular telecommunication system (phase 2+); Transmission planning aspects of the speech service in the GSM Public Land Mobile Network system. (GSM 03.50) Table 3: UK Code of Practice ND1406:1997/10 Document (NCOP040 ( TG 23)) PNO-IG/NNTP ( study group 29) Description A Voluntary Code of Practice for the Design of Private Telecommunication Networks National network timing plan 3 Definitions, symbols and abbreviations 3.1 Abbreviations ADSL BIP CATV CPE CRC Asynchronous Digital Subscriber Line. Bit Interleaved Parity. Cable television. Customer Premises Equipment. Cyclic Redundancy Check. CT2 Cordless Telephony (system 2). DECT ETS ETSI GPS GSM GSTN HRX ISC ISDN ISP ITU-T LD-CELP Digital European Cordless Telephony. European Telecommunication Standard. European Telecommunications Standards Institute. Global Positioning System. European Digital Cellular Telecommunication System, Global system for mobile communications. Global Switched Telephony Network. Hypothetical Reference Connection. International Switching Centre. Integrated Services Digital Network. Internet Service Provider. International Telecommunications Union - Telecommunications Standardisation Section. Low Delay Code Excited Linear Prediction.

13 13 ND 1701 V1.5.2T T( ) NTP OLR PBN PBX PCM PDH ppm PRC Network Termination Point. Overall Loudness Rating. Packet (or Cell) Based Network. Private Branch Exchange. Pulse Code Modulation. Plesiochronous Digital Hierarchy. Parts per million. Primary Reference Clock. PSTN Public Switched Telephone Network. qdu RLR SDH SLR STM-N TACS TU12 UMTS xdsl VDSL Quantising Distortion Unit. Receive Loudness Rating. Synchronous Digital Hierarchy. Send Loudness Rating. Synchronous Transport Module (level N). Total Access Communications System. Tributary Unit (first level, second rate). Universal Mobile Telecommunications System. Digital Subscriber Line, where x represents different bandwidths structures. Very High Speed Digital Subscriber Line. 4 Responsibilities and general criteria 4.1 Responsibilities The primary responsibility for determining compliance with these recommendations lies with the operator selected by the customer to carry the call. For example, if a customer dials a number preceded by the access code for, say, 'Commsco', then Commsco is responsible for the quality of the call. It is the responsibility of Commsco to negotiate appropriate interconnect agreements with other operators to ensure adequate network performance for its customers. 4.2 General criteria For typical PSTN voice connections over digital networks, voice transmission quality depends primarily on received level, delay, echo and quantising noise. Talker echo is a particular source of voice transmission degradation as a result of increased delays in digital networks. For significant delays, control of talker echo is achieved by the addition of echo cancellers. Moreover in the case of voice band data transmission, quality is particularly sensitive to quantising noise, and saturation of the encoder by modem signal levels will result in higher error rates. Consequently, modem signal levels should be controlled by the voice band data terminal to ensure proper levels at the encoder. Perceived call quality is also affected by the overall transmission delay, where excessive delay can make conversation difficult. This is in addition to echo constraints.

14 14 ND 1701 V1.5.2T T( ) In the case of a voice connection with 4-wire (where separate channels are used for each direction of transmission) digital access to either a 4-wire end-user terminal or a digital private network, echo can only occur as a result of acoustic coupling or crosstalk within the terminal or handset. The digital terminal design requirements will therefore need to provide sufficient talker echo control to preclude the need for network-based echo control devices. The performance of terminal equipment is an important factor in end-to-end quality. This is particularly true for a fully digital environment, where the characteristics of the end-user terminal essentially determine the overall performance. The key transmission planning characteristics of end user terminal equipment are send/receive loudness ratings, quantising distortion and (for 4 wire equipment) delay and echo loss. Current European requirements for these parameters are included in a number of documents. Examples include: ISDN (Telephony) (ETS ) CT2 (EN ) DECT (I-ETS ) GSM/PCN (GTS 03.05) Private Networks (ETS ) Private Networks NCOP040 PSTN Voice Access (Draft EN ) PSTN Telephones (ES ) Network Operators should use the relevant ETSI requirements or quidelines to become aware of likely terminal equipment characteristics. Note: There are no longer any mandatory type approval requirements for CPE that have to be met before equipment can be placed on the market and put into service. Nevertheless, it is necessary to make some assumptions that CPE will behave in a similar manner to that specified in voluntary standards in order to make recommendations on suitable network performance limits. The following principles were used in developing this standard: (1) For end-user services accessing operator networks through digital access, no loss is inserted in the network. Therefore, when both access connections are digital, a digital bit stream applied at the users terminal will be transmitted without modification to the far end users terminal. (2) Digital bit streams are transported between switches without modification except where such modification occurs intentionally as a function of a service. (3) For digital access, it is assumed that loss or level control is incorporated as part of the end-user voice terminal, voice-band data terminal, PBX or private network, and hence is not required in the public network. (4) For analogue access lines, it is desirable to insert network loss, where required, as near to the end-user terminal as possible. 5 Reference connections 5.1 Generic The Reference Connection Elements diagram (figure 5.1) shows the building blocks that can be used to construct an end to end network. Annex A gives some examples of how these connection elements can be chained together to describe end-to-end connections. The body of this document describes planning guidelines for: a "traditional" PSTN/ISDN/GSM network built of copper, fixed radio and GSM access connection elements, circuit-switched core network connection elements and the international GSTN;

15 15 ND 1701 V1.5.2T T( ) the emulation of the PSTN service using new, packet-based technologies. Sections 7 and 8 include guidelines that should be carefully considered when incorporating the following new technologies into the UK PSTN: xdsl access cable modem access UMTS access packet- or cell-based core networks hybrid packet- or cell-based/pstn core networks 5.1 VPN Figure 5.1: Reference Connection Elements An increasing configuration is the use of Virtual Private Networks (VPN) to access the trunk network. From a performance viewpoint this can be described in terms of an addition to the generic reference model combining allowances for a Private Network and the conventional local access. A VPN connection reference model is illustrated in Figure 5.2. This is applicable also to CATV and ISP.

16 16 ND 1701 V1.5.2T T( ) Conventional private network access to Core Network VPN access to Trunk Network Figure 5.2: VPN Network performance design standards reference model 6 Transmission levels 6.1 General issues Transmission levels on multi-operator connections are of particular importance as they impact on a number of performance areas: end to end transmission loss as seen by customers e.g. the loudness of telephony connections. signal levels sent to line level of talker-echo on telephony connections. stability margins on connections performance of echo control devices. This section concentrates on the recommended values for transmission loss, stability and signal level sent to line. Echo control issues are covered in Section Telephony connections Traditionally end to end transmission losses for telephony connections have been specified in terms of Overall Loudness Rating (OLR) which take into account performance of telephone instruments and hence represent the acoustic to acoustic performance as seen by customers. ITU-T recommendations in the G-series include maximum long term and short term objectives for values of OLR. International network planning has required an apportionment of the end to end OLR values into allowances for national extensions and the international transit network. The international apportionment principles can also be applied to the current UK environment where end to end performance may be determined by the networks of several operators. As the inter-exchange networks of UK operators are likely to use digital transmission and switching (thus adding zero loss to the overall connection), the following simplified model can be used for transmission loss planning.

17 17 ND 1701 V1.5.2T T( ) OLR can be sub-divided into SLR (Send Loudness Rating) and RLR (Receive Loudness Rating) where: OLR=SLR + RLR db SLR and RLR represent the transmission losses between the acoustic input/output points and the digital bit stream of the switched network of the two directions of transmission. Recommended values for SLR and RLR are also included in the G-series Recommendations as shown in Figure 6.1. Send Loudness Rating Receive Loudness Rating 7-9 db mean, 20 db max. 1-3 db mean, 9 db max. ANALOGUE DIGITAL ANALOGUE Overall Loudness Rating 8-12 db mean, 29 db max. LOUDNESS RATINGS -RECOMMENDED OBJECTIVES AS DEFINED IN G.111. Figure 6.1: Loudness ratings

18 18 ND 1701 V1.5.2T T( ) Y Stability loss Stability loss X ANALOGUE DIGITAL ANALOGUE Stability loss (X-Y and Y-X) > 6dB Figure 6.2: X Stability loss 6.3 UK Loudness rating values (OLR, SLR, RLR) Achieving the OLR objectives for multi-operator calls within the UK requires agreement of SLR and RLR objectives. It is recommended that switched connections within the UK should meet ITU-T recommended values for SLR and RLR as shown above. Meeting these objectives will also ensure that UK international access connections are consistent with the international transmission loss plan. Demonstration of compliance with these objectives will require the responsible network operator (see sub-section 4.1) to be aware of the contribution to SLR/RLR from the customer equipment as well as other operators' networks. Customer equipment requirements (see ETSI documents) are listed in Section 4 (see also the note in section 4.2). It should be noted that cellular network operators may employ digital level control in the mobile switched network to adequately control SLR and RLR values. 6.4 Voice-band data connections Compliance with the above recommendations for telephony is likely to result in acceptable values of transmission loss for most voice-band data services. 6.5 Signal levels sent to line Signal levels on voice services will normally be adequately controlled by limiting the minimum value of outlined in Recommendation G.111. SLR as Recommended maximum power levels for non-voice signals (e.g. data transmission) are given in ITU-T recommendation V.2 (power levels for data transmission over telephone lines) and TBR Stability The stability performance requirements for multi-operator networks are aimed at ensuring that the end-end connection does not enter a state of oscillation during the various stages of a call (set-up, clear-down, and the information transfer phase). Stability is determined by a number of network factors such as transmission loss, impedance matching and design of handsets. However the overall requirement can be expressed in terms of a 'stability loss' specified at the boundary between 2 networks. The principle of this measurement is outlined in ITU-T Recommendation G.122 (Influence of

19 19 ND 1701 V1.5.2T T( ) national systems on stability talker-echo and listener echo in international connections). In the UK environment the stability loss presented by networks should meet the principles of the requirements in Sections 2 and 3 of Recommendation G.122. These requirements will be met if the attenuation between the input and output ports at a network boundary is at least 6 db at all frequencies in the range 200 Hz - 4 khz for all connections, (see Figure 6.2). Demonstration of compliance with these objectives will require the network operator responsible for quality (see subsection 4.1) to be aware of the stability loss contribution from customer equipment and other operators' networks. Recent changes to the approval standards for terminal equipment mean that there is a risk that on very short local lines the stability loss could be a low as 8 db (assuming a 6 db contribution from the network). 7. Delay and echo 7.1 General The introduction of digital switching and transmission systems into the public network (including cell-based and packet-based technologies) has caused transmission delay and echo control to become increasingly significant issues. There are two main reasons for this: 1. Digital systems generally have higher inherent delay than analogue systems. This means that delay and echo can be experienced on trunks where it was not previously a problem. 2. Digital networks generally have a lower transmission loss than analogue networks. This can result in a higher level of echo being returned to the talker. The recommended approach to minimise delay and echo effects is to specify limits for end-to-end delay and echo path loss that will ensure that delay and echo are not problems. However, some national calls within the UK will inevitably exceed the delay limits, and echo control will be required for these calls. The deployment of new switch and transport technology will require increasing use of echo control. Echo control can be achieved either by the use of echo cancellers or a high echo path loss. Longer delays, even when echo is effectively controlled, need to be minimised as transmission delays themselves are a source of quality of service degradation. 7.2 Connections without echo control For voice applications on UK connections not employing echo control, the NTP - NTP one-way delay should be less than 15ms for at least 95% (see Note 1) of calls. Assuming that customer network delays at each end of the link do not exceed 5ms (as recommended in ETS , EG and EG ), then the end-user to end-user delay should normally be less than 25ms. This aligns with ITU-T Recommendation G.131 that sets 25ms as the limit above which active echo control will be required. It should be noted that the introduction of cell-based or packet-based technology (such as VoIP) into customer networks could result in delays exceeding 5ms in the CPE. For the small proportion of calls that exceed the 15ms limit, an absolute limit of 25ms is recommended. Users may consider such calls to be unacceptable, since customer network delays could raise the end-user to end-user delay to more than 30ms. The delays on international access connections need to be considered as they will have an impact on echo canceller deployment in the international network. The overall objective is for the NTP to ISC delay to be less than 9ms for at least 95% of calls, with an absolute limit of 12ms. In situations where calls have to be re-routed around failed sections of the network, it is acceptable for the proportion of calls meeting recommended delay limits to fall below 95%. However, the absolute limits given above should not be exceeded. The delay limits given above are based upon the assumption that transmission losses are set as described in Section 6

20 20 ND 1701 V1.5.2T T( ) and that the echo path loss is at least 15dB. Since the implementation of the RTTE Directive in April 2000, individual CPE product performance approval standards are no longer applicable. Hence this means that the echo path loss value specified in the approval standards for non-handset voice terminal equipment cannot be relied upon for all types of CPE. In the case of private networks (made up of a number of CPE products), such a problem may not arise and consultation with the private network operator is recommended. There is a risk that on very short local lines the echo path loss for these devices could be a low as 10 db (assuming a 6 db contribution from the network). A digital interconnection between two digital networks should not introduce any gain or loss, so the transmission level of a call crossing between the two networks will be determined at the periphery of the access network (for digital access, the transmission levels will be set by the CPE). The use of in-line transmission loss (i.e. loss that is encountered by the legitimate signal as well as the echo) is not recommended as a means of controlling echo. Note 1: Compliance with the 95% objectives should be able to be demonstrated by the operator responsible for the quality of the call. In order to minimise the risk of some customers always encountering unacceptable performance, the objectives should be applied on a catchment area basis. Using the 'Commsco' example (see 4.1) 95% of the Commsco's national calls originating from a particular catchment area served by the Commsco switch should have NTP-NTP delays of less than 15ms. 7.3 Connections with echo control Where echo control is provided it is generally true that the performance of the connection for interactive traffic (either voice or data) is degraded as delay increases. It should be an objective when planning connections with echo control to minimise the delay National connections with echo control Connections within the UK requiring echo control fall into three categories: 1. Connections over the fixed network with delays sufficiently long to potentially cause annoying talker echo and delay. 2. Connections between mobile digital networks or wireless access networks and the fixed network. 3. Connections between digital mobile network users or wireless access networks users. (Echo control is by acoustic loss in the case of connections between digital mobile network users). With connections over the fixed network within the UK it is recommended that 95% of national connections with echo cancellation should meet a 125ms one way delay limit and that no connections should exceed 150ms one-way delay. Similarly with connections between digital mobile networks or wireless networks and the fixed network within the UK it is recommended that 95% of call connections should meet a 125ms one-way delay limit and that no connection should exceed the 150ms one way delay limit. With connections between mobiles on digital networks or users of wireless access networks within the UK it is recommended that 95% of calls have a one-way delay of less than 215ms and that no connection should exceed 230ms one-way delay. See Note 1 above and Note 3 below on compliance with 95% objectives. Note 2: These values will need to be reviewed for the case of national satellite connections Special considerations for non-tdm networks The following one-way delay objectives are recommended for the component parts of UK connections for 100% of call cases. There may be access technologies other than mobile and wireless which offer advantages to customers but which add more than the delay allocated to an access portion. It may be necessary to add specific delay objectives for such technologies in the future. The delays presented below are a single figure for each network element. It is recognised that

21 21 ND 1701 V1.5.2T T( ) some processes are unidirectional and some configurations lead to different delays in one direction compared to another. However, the customer perception of delay is based the interaction between the two callers i.e. it is based on the round trip delay. So although a unidirectional analysis of delay can be made, it may be possible to show in some cases that only an evaluation of the average of the two directions is necessary. Fixed Access (between the UNI and the switched network serving the access): Traditional copper pair 3ms. Digital Mobile or Fixed Wireless Access (between the point of A/D conversion and the switched network serving the access): 104ms for GSM access and fixed wireless access; 111ms for 3G access. This value excludes any delay associated with processing in the terminal for handsfree operation. Originating Switched Network (between ingress from originating access network and egress to interconnection with subsequent switched network operator): The objective for an Originating Switched Network is equal to the national network delay objective (see ). Transit Switched Network (between interconnection from preceding switched network operator and interconnection with subsequent switched network operator): The objective for a transit Switched Network is equal to the national network delay objective (see ). Terminating Switched Network (between ingress from interconnection from preceding switched network operator and egress to terminating access network): The objective for a Terminating Switched Network is equal to the national network delay objective (see ). Originating/Terminating Switched Network (between ingress from originating access and egress to terminating access): The objective for an Originating/Terminating Switched Network is equal to the national network delay objective (see ). These recommended delay objectives apply to each operator whose equipment forms part of the end-to-end call path. The simplest call path will consist of two access components interconnected by an Originating/Terminating Switched Network. These recommended delay objectives do not include signal propagation time (which is almost negligible for calls within the UK) nor do they include an allowance for the following functions which introduce a small amount of delay but which should occur only once in the call path: Analogue-Digital conversion and Digital-Analogue conversion Echo cancellation The overall effect across all networks of propagation delay, A/D and D/A conversion and echo cancellation should be less than 10ms delay for all UK calls. Each network in a call path shall introduce no more than 1ms of propagation delay for each 100km (direct, point-to-point distance between ingress and egress) the call is carried across their infrastructure. The permissible propagation delay includes any network re-routeing (e.g. for number portability) but excludes the effects of any customer actions, such as call forwarding from the original intended destination of the call. In the case where an operator manages both the access and the originating or terminating switched network components of a connection, their delay objectives may, if necessary, be concatenated and divided between access and switching, bearing in mind the need to keep delay low. Where access technologies with intrinsically low delays are used operators are encouraged to refrain from using the allowance for access components to increase the delay in their switching component The national network delay objective Networks interconnected using TDM Operators should endeavour to keep delay to a minimum. For networks which are interconnected using TDM the maximum one-way delay across a packet-switched network should not exceed 35ms (excluding propagation delay). It is recognised that the maximum one-way delay value will result in perceptible degradation for certain UK call scenarios. The use of an IP-based interconnect will reduce the number of delay intensive functions and consequent degradations. See and ND1704 for further guidance.

22 22 ND 1701 V1.5.2T T( ) It is recognised that some packet-based networks may have already been deployed and that some operators may have already committed themselves to procure equipment which cannot achieve the delay target. The delay target cannot be retrospectively applied to such networks but in the interest of maintaining PSTN quality it is recommended that they move towards this figure as soon as possible. Operators should be aware that in the event of excessive delay causing QoS problems to customers the primary responsibility for resolving the delay problem will lie with any operators which have deployed networks with delays significantly above the target delay. Networks unable to resolve internal network delay issues would be expected to migrate to packet based interconnects or aim to mitigate problems by adopting alternative network interconnect routeing or topology. Note 3: The introduction of islands of packet-based technologies into the UK PSTN may have an effect on perceived QoS. As these technologies become more widespread the probability of a call undergoing multiple packetisations increases and the accumulated impact on QoS, in terms of delay and speech quality, may be perceived by some users. It is possible that calls between a very limited number of certain number pairs may encounter reduced quality during the earlier stages of PSTN evolution towards IP. Calls involving complex routings are the most likely to be affected. Network operators should discuss the potential impact of any planned introduction of new technologies, particularly IP, with interconnected operators in order to minimise any potential adverse effect on call quality. Section 8 describes a set of general planning guidelines that should be carefully considered when incorporating new technologies into the UK PSTN Networks interconnected using IP Operators should endeavour to keep delay to a minimum. Using IP interconnects for IP networks reduces the possibility of having multiple delay-intensive packetisation functions. The IP interconnect may also reduce the number of dejitter buffer instances, although as packet delay variation is thought to be additive this requires that the remaining dejitter buffers are larger and there may not be a reduction in the overall delay. Specific delay objectives for Next Generation Networks interconnected using IP can be found in ND Hybrid TDM/IP Call Paths By considering the delay associated with specific functions (e.g. packetisation, switching and packet dejittering) it is possible to allocate objectives for different scenarios of originating, terminating and transit networks with both IP/NGN and TDM interconnect at the same time. The aim being to minimise delay as UK networks migrate to the all NGN scenario described in ND1704. In order to help minimise delay the following general rule should be applied: For a given call path where a CP s network is interconnected to other CP s networks exclusively by NGN interconnect the performance guidance in ND1704 is applicable. For a given call path where a CP s network is interconnected only with TDM, or a combination of TDM and IP/NGN interconnect the performance guidance in ND1701 is applicable Networks interconnected using other technologies Delay objectives for networks interconnected using technologies other that TDM and IP are currently outside the scope of this document International connections with echo control International connections originating or terminating within the UK requiring echo control fall into two categories: 1. Fixed network originated connections. 2. Mobile network or wireless access network originated connections. The maximum one way delay for the international connection will depend on the nature of the destination network e.g. fixed, digital mobile or wireless access and the transmission media used e.g. satellite or cable.

23 23 ND 1701 V1.5.2T T( ) With international connections using echo control it is recommended that connections with a one way delay exceeding 400ms should only be provided under exceptional circumstances such as back-up circuits provided via satellite to maintain service in the event of cable circuit congestion or failure. In addition some destinations may only be reached via satellite routeings, opening up the possibility for long delay calls exceeding ITU-T G.114 guidelines. The UK components of international connections should follow the guidance given above regarding delay. 7.4 Echo Control Echo cancellation should be employed in any network that uses packet- or cell-based technologies as the delay introduced by such technologies is likely to push the end-to-end delay above 25ms. Echo cancellers compliant with the requirements of ITU-T Recommendation G.168 should be employed. The presence of an echo canceller in a call path should be signalled to adjacent networks in accordance with ITU-T Recommendation Q.115. Ideally all echo cancellers should be disabled with the exception of the two closest to the two potential sources of echo. Annex B illustrates a number of scenarios in which echo is required and shows which echo cancellers should be disabled by signalling. Note: Existing signalling systems do not enable the presence of echo cancellers in private networks to be signalled to public networks. If G.168 compliant echo cancellers are used in private networks then the potential presence of two echo cancellers working in the same direction of a call path will not degrade speech quality. In general, the tail capacity must be at least 64ms to prevent any potential failure to cancel echo in some routeing scenarios. As an exception to this, the tail capacity can be smaller when the echo path is known to be less than 64ms (e.g. when the echo canceller is cancelling acoustic echo in a terminal). Circuits with echo suppressors can be connected with circuits equipped with echo cancellers without additional performance degradation caused by the canceller; however, the overall performance will be limited by that provided by the poorer performing device. In some cases echo cancellers conforming to G.165 may have already been deployed. The interaction of G.165 and G.168 echo cancellers may need to be the subject of further study. Note that much new detail regarding the use of echo cancellers is provided in ITU-T Recommendation G Planning guidelines for the incorporation of new technologies into the UK PSTN Careful consideration should be given to the effect on QoS (notably delay) when planning the introduction of new technologies into the PSTN. The guidelines given in this section should be taken into account in order to minimise the impact on QoS of the introduction of any new technologies. This document only addresses the performance limits of public networks (i.e. the performance between a pair of Network Termination Points). The existence of private networks cannot be ignored in network planning because they will have an effect on end to end impairments. Historically the NTP-NTP delay performance of the PSTN was below 15ms and private network operators have been able to use this low delay to allow the implementation of low bit rate codecs and IP technology and still achieve acceptable quality for calls from the private network to the PSTN. As IP technology is deployed in the PSTN, private network operators should bear in mind that the delay in the PSTN will increase. The limits for PSTN delay set out in sections and of the current document should be taken into consideration when planning access and private networks. Note: The ITU-T s E-model (as contained in Recommendation G.107 and G.108) provides transmission planning guidance based on the perceived effect of impairments, including those of delay and codecs. Note: In the context of section 8 the term jitter refers to IP packet jitter (also called packet delay variation ). This should not be confused with jitter in the sense of the timing of digital signals, as discussed in section 9.

24 24 ND 1701 V1.5.2T T( ) 8.1 General Rules Rule 1. In order that the number of "packetisations", and therefore delay, can be kept to a minimum, networks carrying voice over IP technology should ideally be interconnected using IP. Note: -The availability of TDM and IP or other broadband interconnects within the UK is driven by other considerations as well as performance. Timescales for the implementation or withdrawal of interconnect products is well beyond the scope of this document. Rule 2. Care should be taken to ensure that the performance of PSTN traffic is not adversely affected by other traffic types. The network should deliver a very low latency and jitter and preserve PSTN grade of service (GoS) even under high or rapidly fluctuating load and/or mix of other traffic types. 8.2 Delay and Packet Jitter Rules Rule 1. Transmission delay should be minimised. See section 7.3 for detailed guidance. Rule 2. IP packet jitter should be minimised. Note: The specification of packet jitter objectives is for further study.operators should be aware of the contribution that jitter in packet streams entering the public network from a private network makes to the performance of the overall end-to-end connection. Rule 3. IP jitter buffer delay should be minimised. Note: There is a trade-off between this parameter and packet loss, with shorter jitter buffer delays potentially resulting in higher packet loss. This should be borne in mind when optimising these two parameters. Rule 4. A de-jitter function is required at the point of transition from a packetised voice bit stream to a synchronous voice bit stream. For calls within the UK, de-jitter functions should not be applied on a per network element basis or at the boundary between two IP-based networks interconnected with IP. Note: There may also be a need use a de-jitter buffer at points where the codec type or speech sample size are different to those specified in 8.3 Rule 1 and Rule 2. ITU-T Rec. Y.1541 allows for up to 50ms of jitter end-to-end in the most stringent QoS Classes. Jitter of this magnitude may be present on calls entering the UK. Rule 5. In order to help minimise delay, adaptive jitter buffers are recommended. For calls involving fax, dial-up modem and ISDN clearmode the jitter buffer should be fixed at an optimum value on a per connection basis so as to avoid data loss during dynamic changes of the jitter buffer size. For ISDN clearmode the fixed jitter buffer size should be established on call set-up (i.e. before any user data is sent). For fax and other modem calls the call will start as a voice call with an adaptive jitter buffer, the jitter buffer should then train to a fixed setting. 8.3 Codec Rules Rule 1. The G.711 A-law codec, without silence suppression, shall be used as the default interoperability option for interconnect purposes because it has the lowest impairment value and therefore it allows more delay for a given voice quality level. Note: Connections can be established using other codec pairs, either by bilateral interconnect agreement or end-to-end per-call negotiation. Similarly other codec pairs can be employed within one operator's network or in access networks. In order to guarantee adequate QoS codecs other than G.711 should only be employed in situations where there is certainty over the routing of calls. Operators should be aware of the potential impact that codecs other than G.711 can have on perceived speech quality. Rule 2. The speech frame size and the number of speech frames per packet should be minimized when using IP technology. In order that the effects of packetisation on delay can be kept to a minimum, for voice carried over IP technology a speech frame size of 10ms for G.711 should be supported for UK NGN-interconnect. Note: Speech frame sizes of other than 10ms may be more appropriate for certain codec types. Operators should be aware of the potential impact that the additional delay caused by speech frame sizes of more than 10ms can have on perceived speech quality Rule 3. Operators should be aware that silence suppression may adversely affect the perceived quality of a call and should not be used except in situations where there is certainty over the routing of calls. Rule 4. The following add delay and or coding impairment and should be avoided where possible: multiple packetisations Note: The benefit of avoiding multiple packetisations is only gained if networks use the same speech frame size and number of speech frames per packet. See Rule 2 above.